At the start of an injection molding process, thermoplastic pellets are fed by a hopper into a heated barrel and driven to the end of the heated barrel by a reciprocating screw. The thermoplastic pellets melt into a molten thermoplastic material, and shots of thermoplastic material are injected through a nozzle. The molten thermoplastic material then flows through either a cold runner or a hot runner to the gates of each individual cavity of a mold. After entering the gate, the molten thermoplastic material fills a mold cavity formed between two or more sides of the mold held together under pressure by a press or clamping unit. Once the shot of molten thermoplastic material is injected into the mold cavity, the reciprocating screw stops traveling forward. The molten thermoplastic material takes the form of the mold cavity and cools inside the mold until it solidifies. Then the clamping unit releases the mold sides and the finished part is ejected from the mold.
The injection molding process may incorporate molds which manufacture multiple parts for each representative cycle, each of the multiple parts being formed in a distinct mold cavity. The mold may be identified by the number of parts that it manufactures in each cycle. For example, if a mold manufactures eight parts at a time, it is referred to as an eight-cavity mold. The runner system of a mold includes channels which deliver plastic to each individual cavity. A runner system is referred to a cold runner system or a hot runner system.
Whatever the particular cavitation of a mold, or type of runner system, distributing the molten plastic evenly to each individual cavity is desirable in order to prevent some cavities from filling too soon or too late, or at too high a pressure or too low a pressure, as this would have a potential effect on the quality for the parts produced. Specifically, the flow of plastic to a particular cavity could occur before or after the overall average fill time of the group, resulting in inconsistent quality of parts. For example, if cavity 1 fills first or at a higher pressure, followed by cavities 2-7 filling simultaneously or at median pressure, followed by cavity 8 filling lastly or at a lower pressure, the part formed in cavity 1 may have a defect known as flash or may be heavier or dimensionally larger part than the parts produced in cavities 2-8 Likewise, the part formed in cavity 8 may have a defect known as a short-fill or may be a lighter or dimensionally smaller part than the parts produced in cavities 1-7.
A cold runner system is a method of delivering plastic to each cavity of a mold in which plastic cools and solidifies into the shape of the desired part as defined by the mold cavity and into the shape of the channels that were designed to distribute the plastic melt to each mold cavity. The cold runner system is geometrically designed to account for the flow lengths and pressure drops to each gate location for each mold cavity in order to balance the fill of each mold cavity as equally as possible. However, the plastic from the runner system needs to be discarded as either scrap or needs to go through an additional process of being recycled (regrind) and re-introduced to the molding process at another time. This adds an overall increase in cost to produce any given plastic part using a cold runner system.
A hot runner system, similar to a cold runner system, is geometrically designed to account for the flow lengths and pressure drops to each gate location for each mold cavity. In addition, hot runners are thermally designed with heating zones which may be controlled with a heater and thermocouple combination to keep the plastic within its channels molten so that it may be used immediately for the next cycle. This eliminates the scrap and regrind that are associated with a cold runner system. Although hot runners are geometrically and thermally designed to distribute the flow of plastic evenly to each cavity of the mold, the effects of differential of heat transfer throughout the hot runner system as well as variation of melt due to laminar flow effects often necessitate changing temperatures in the hot runner system through trial and error. The change in temperatures is routinely accomplished using an output such as part weight or part dimension to determine which zones need to be changed in order to achieve optimal balance of fill to each cavity.
Ideally, sensors for monitoring an injection molding process would be indirect, easy to install, and inexpensive. Direct sensors, such as sensors placed within a mold cavity, leave undesirable marks on part surfaces. For example, while demand for injection molded parts with high gloss finishes has been increasing, direct sensors positioned in the mold cavity have a tendency to mar the high gloss finish of the parts, requiring post-molding operations to machine or otherwise mask or remove the marred regions from the parts. As a result, indirect sensors that are not located in the mold cavity are preferable. Additionally, when the molding system is being used to make products for medical applications, contact between a sensor and the thermoplastic material may be prohibited.
More recently, strain gauges have been placed on a mold surface, within a nozzle adapter, or elsewhere within an injection molding apparatus, in order to measure how strain at the measured location changes over the course of a standard injection molding process. For example, a strain gauge sensor placed on the exterior of the mold surface adjacent to a parting line of a mold, as described in co-owned U.S. Patent Application No. 62/303,654, “External Sensor Kit for Injection Molding Apparatus and Methods of Use,” the entirety of which is hereby incorporated by reference, is able to sense the surface strain changes on the mold surface that occur over time as a result of the closing and opening forces. In response to surface strain changes, the strain gauge sensor emits an electrical signal, typically in the range of −10 to 10 Volts. The signal emitted by the strain gauge sensor is received and used by a controller to approximate one or more conditions within the mold, such as the pressure within the mold cavity or the location of the melt flow front. In certain molds in which the ratio of the length of the flow channel to the thickness of the molded part is great, i.e. molds having a high length-to-thickness (L/t) ratio, the pressure at the melt flow front may be approximated based on the signals emitted by the strain gauge sensor(s).